This application claims priority to Japanese Application No. Hei 10-321158, filed on Nov. 11, 1998, which is incorporated in its entirety.
1. Field of the Invention
The present invention relates to a radio communication system for data communication and a computer program product stored on a storage medium for controlling an operation of the radio communication system.
2. Discussion of the Background
In a background communication technique in a radio communication system for transmitting mainly a voice signal, communication is performed only in accordance with the intention of calling, regardless of whether or not there is noise. This communication also does not consider the conditions or characteristics of a radio wave propagation path used for communication, because communication is required to be performed in real time. In many cases, data communication is also performed without considering whether or not there is noise, because the data communication is based on a voice communication technique.
Neglecting the conditions of the radio wave propagation path results in a large reduction in communication efficiency if there is large noise. In voice communication, an instantaneous error does not cause a serious problem (although it can cause noise during speech), because such a type of noise does not exert a great influence on the content of the speech.
On the contrary, data communication degradation in the signal can cause a serious problem. Unlike voice communication, an error is not allowed in data communication. Therefore, if an error occurs in a radio communication system, a receiving end sends an ARQ (automatic repeat request) signal to a sending end. In response, the sending end again transmits the data. The retransmission of data is performed until data is successfully transmitted without error. However, retransmission of data results in an increase in the amount of transmission required to transmit the same data. As a result, the throughput decreases.
In a two-frequency simplex radio communication system, there is a possibility that a collision with data transmitted from another mobile station occurs when an ARQ signal is sent from the receiving end (mobile station). This results in a further reduction in the throughput. Thus, it is important to reduce the number of times data is retransmitted in response to ARQ signals.
A waveform equalization technique for improving the digital error rate is disclosed in Japanese Patent Laid-Open No. 7-221804. In this technique, the transfer function of a transmission path is estimated and the inverse transfer function is calculated. The waveform of a signal containing distortion or noise is corrected into the original form using the inverse transfer function.
However, this technique needs an extremely large amount of calculations, and thus this technique does not meet the requirement that the radio communication device used in the radio communication system be small in size and consume small power.
Another problem of this technique is that when there is fast fading, linear demodulation is difficult and sufficient equalization cannot be obtained.
Another known technique of improving the digital error rate is to add a redundant code to data so that an error can be corrected by redundancy. In this technique, to obtain a high correction ability, it is required to use a large number of redundant codes. This results in a reduction in the transmission efficiency.
Japanese Patent Laid-Open No. 7-87011 discloses a radio communication system and a radio communication device in which communication between a mobile station and a base station is performed using a directional antenna. In this radio communication system, control of the base station needs a complicated process, and expensive facilities are required. Furthermore, because this technique employs a communication procedure including a step in which the base station analyzes the status of the mobile station, communication becomes impossible during a period in which analysis is performed.
Diversity transmission techniques such as space diversity, time diversity, and bus diversity are also known in the art to improve the digital error rate when communication is performed under a poor radio wave propagation path conditions. Among various diversity techniques, space diversity is most powerful. However, this technique has difficulties in achieving a small size, low cost, and low power consumption.
Japanese Patent Laid-Open No. 8-274781 discloses another technique of improving the digital error rate under a radio wave propagation path. In this technique, a service area in which an field strength high enough to receive information service is obtained is explicitly designated, for example, by coloring the floor in that area so that users can easily distinguish the service area. However, it is generally difficult to designate service areas in public locations in such a manner. In particular, this technique cannot be applied to a radio communication system having a wide service area such as several km. Besides, this technique needs an additional investment. Still another problem of this technique is that waveform distortion due to, for example, multipath delay cannot be detected from the field strength.
Accordingly, one object of the present invention is to solve the above-noted and other problems.
Another object of the present invention is to provide a radio communication device, a radio communication system, and a storage medium on which a program for implementing the functions of such a radio communication device or radio communication system is stored, whereby a high transmission efficiency can be maintained without having to perform a large amount of complicated calculations.
To solve these and other objects, the present invention provides on one example a mobile station of a two-frequency simplex radio communication system in which communication between a base station and each mobile station is performed in such a manner that the base station continuously transmits packet data and each mobile station transmits burst data. The mobile station includes a radio wave propagation path condition detecting mechanism for detecting a condition of a radio wave propagation path used for communication, and a communication control mechanism for adaptively controlling communication in accordance with the condition of the radio wave propagation path detected by the detecting mechanism.
The communication control mechanism adaptively selects a transmission mode when the condition detected by the detecting mechanism indicates that transmission or reception cannot be obtained in within a predetermined time period, and notifies the base station of the adaptively selected transmission mode. In addition, the mobile station receives data transmitted from the base station after the communication control mechanism notifies the base station about the adaptively selected transmission mode.